Thanks for help, I think 2.5 for max and empower is correct. Also, you really dont need 15 fireballs in a row, I just pointed out that one could cast at that rate. Its comparable to saying the GAU-8 Avenger auto cannon has a rate of fire of 4200 rounds per minute. Of course the A10 never fires that many rounds and only carries a fraction of that amount as ammo.
Casting a fireball vs firewall is exactly what you want to use if you prefer to perform better levels 6 - 10 (other spells at higher level). Further, never stated you didnt need WOF. Again, as I pointed out, it is situationally very useful, or for those few times where you really want to pump up your DPS. However, a DPS approach will net you more kills faster. This can be true even in undead quests (and is true in all quests prior to high level play) where the wall of fire performs best. I am not sure your point, are you claiming a Wall of fire has higher first damage or DPS? If not, then how can you claim it is a better approach?
In first shot kill situations AOE can be cheaper than wall of fire. In multiple shot cases it cost more. Is this surprising? If it cost less there would never be a reason to use wall of fire. At 8th level you are talking something around 20 maximized fireball shots between shrines, or more than twice that if you don't maximize the spell (which is often all that is needed). There is typically no need for half that many.